diff --git a/requirements.txt b/requirements.txt
index 8130bed..948e7b6 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -13,3 +13,4 @@ requests>=2.9.1
 # Python3 compat
 future>=0.15.2
 configparser>=3.3.0; python_version < '3'
+Sphinx<2.0; python_version < '3'